D lines (of sodium)

The D lines are two close prominent lines in the yellow region of the spectrum of neutral sodium at wavelengths of 588.9950
and 589.5924 nanometers (approx. 5890 Å and 5896 Å). They were
labeled as feature D in the solar spectrum by Joseph von Fraunhofer and are conspicuous in the light of stars similar in temperature to the
Sun (see Fraunhofer lines). They
also show up in the spectra of remote stars due to absorption by sodium atoms in the interstellar medium.

The energy level diagram (right) shows
that the D lines are emitted by electron
transitions from the 3p to the 3s levels of sodium. The line at 589.0
nm has twice the intensity of the line at 589.6 nm.